In the course of erecting steel structures such as multi-story buildings, a thick coating of a fireproofing material is commonly applied to metallic structural elements. While several types of formulations have been applied for this purpose over the years by means of a variety of techniques, the most successful system so far consists of spraying onto the steel surfaces settable aqueous mixes composed essentially of calcined gypsum, a lightweight inorganic aggregate material of exfoliated vermiculite, a mixture of fibrous materials such as a high wet bulking cellulosic fiber and glass fiber, and an air entraining agent. A composition of this type is described by Bragg in U.S. Pat. Nos. 3,719,573 and 3,839,059, along with the most desirable application technique, i.e., pumping the aqueous mix and spraying it directly onto the steel.
In order to be suitable for such use, coating mixes, both in the wet and set state, must possess a number of crucial properties. They must be able to hold the large quantity of water that renders them capable of being pumped easily and to great heights, yet they must retain a consistency sufficient to prevent segregation or settling of ingredients during pumping and permit adequate "yield" or coverage of the steel surface at a given thickness. The coating mixes, furthermore, must adhere to steel surfaces, both in the slurried state and in the dry state as well as during a fire. Also, the mix must have a controlling set without exhibiting undue shrinkage which could result in the formation of cracks that would seriously diminish the insulative value of the dry coating.
As intimated earlier, this complex balance of properties has substantially been achieved heretofore by aqueous mixes containing gypsum binder, expanded (exfoliated) vermiculite aggregate and cellulosic fiber. These aqueous mixes can be pumped to great heights and spray applied as an up to 3/4 inch thick coating in a single pass. As the coating sets further layers can be applied thereover.
Exfoliated vermiculite possesses a number of qualities which make it attractive for use as an aggregate in pumpable, settable fireproofing plasters. For example, such vermiculite is of relatively low density and thus remains suspended in the fireproofing slurry. Air trapped within the vermiculite contributes to its good insulating characteristics. Further, the irregular shape and surface contour of each vermiculite particle is believed to trap water as well as the cellulosic and optional glass fibrous components thus aiding the maintenance of a homogeneous slurry.
There has been a need and motivation in the art to formulate new spray applied fireproofing compositions which satisfactorily employ aggregates other than vermiculite while providing the diverse mix of properties referred to above. As described, this need has now been met.